Does the Number of Dental Implants Affect Marginal Bone Loss in the Posterior Mandible?

Statement of the Problem: Marginal bone loss (MBL) is an important factor in dental implant failure. The number of implants may affect MBL. Purpose: The aim of this study was to compare MBL in patients who received two or three implants for replacement of three missing teeth in the posterior mandible. Materials and Method: This prospective cohort study evaluated patients who required replacement of three missing teeth in the posterior mandible with dental implants. The patients were assigned to two groups. In the group 1, the edentulous area was restored with two implants and a pontic while three implants were placed for this purpose in the group 2. The MBL was compared between the two groups at 12 and 24 months after loading. Results: Forty-two implants were studied in group 1 and 36 implants in the group 2. The mean MBL was 0.90±0.12mm in the group 1 and 0.89±0.12mm in the group 2 at 12 months after loading. The mean MBL was 1.00±0.10mm in the group 1 and 0.98±0.10mm in the group 2 at 24 months after implant loading. The mean of MBL was not statistically different between the two groups at 12 months and 24 months (p> 0.05). Conclusion: It seems that the use of two or three implants for replacement of three missing teeth in the posterior mandible is not associated with an increase in MBL.


Introduction
Rehabilitation of the posterior mandible in edentulous patients with dental implants is a commonly practiced treatment [1]. When three teeth are missing, there are two options for their replacement with dental implants: two implants with a pontic and three implants with three fixed prosthetic separate crowns. Each option has advantages and disadvantages [1][2]. The first option (two implants with a pontic) is cost-effective and easily applicable when the mesiodistal space is insufficient. Occlusal overloading may aggravate the marginal bone loss (MBL) [2]. In the placement of three implants, the cost of treatment increases and space management may be problematic. However, increasing the number of implants may help in better distribution of occlusal loads and decrease the MBL [2].
The stability of peri-implant bone is an essential par-ameter for the long-success of dental implants [3]. The dental implant success criteria are complex, but achieving stable osseointegration is a critical parameter in this respect [4]. MBL is a key factor in the success of dental implants. MBL≤ 2mm during the first year after functional loading is considered normal [4]. The search of the literature by the authors revealed no study comparing MBL following the aforementioned two treatment options for replacement of the lost teeth in the posterior mandible. Hence, this study was conducted to address whether the number of dental implants for the replacement of three missing teeth in the posterior mandible affect the MBL ort.

Materials and Method
The authors designed a prospective cohort study. The s- All implants were loaded three months after placement. TS III Osstem implants (Osstem, South Korea) were used. All prostheses were cemented type and split.
One oral and maxillofacial surgeon placed all implants, and one prosthodontist fabricated the implant restorations. Digital parallel radiographs were also obtained during the study period. All radiographs were taken in the same oral and maxillofacial radiology center. Two radiology experts measured the MBL. The MBL was measured at the mesial and distal of implants by comparing the bone level on the digital parallel radiographs taken immediately after loading, and at12 and 24 months later.
When the MBL was different at the mesial and distal implants, the mean MBL was calculated and reported.
The bone level was measured from the alveolar crest to the fixture collar. The patients were assigned to two groups. The patients received three implants in the group 1, and two implants in the group 2. The age and gender of patients and implant diameter and length were the study variables, while the MBL was the outcome of the study. The use of two or three implants was the predictive factor of the study. An inter-examiner reliability analysis was done using the Kappa test to assess the consistency between the examiners.

Statistical Analysis
The statistical analyses were performed using the Statistical Package for the Social Sciences for PCs, version 21 (SPSS Inc., IL, USA). The independent t-test was ap-plied to compare the MBL, and fixture's length and diameter between the two groups. The Chi-square test was used to compare the number of males and females in the two groups. We considered p Values< 0.05 as statistically significant.

Results
Twenty-one patients with total 42 implants were studied in group 1 and 12 patients with total 36 implants in the group 2 ( Table 1). The mean age of patients was 41.  Table 2). The mean MBL was 0.90±0.12mm in the group 1 and 0.89±0.12mm in the group 2 at 12 months after loading (Figure 1). There was no significant difference in the mean MBL between the two groups at 12 months after loading (p=0.63). The mean MBL    Figure   2). Analysis of the data did not demonstrate any difference in the mean MBL between the two groups at 24 months after loading (p= 0.35; Table 3). The interexaminer reliability was kappa=0.52 (p=.0.008) at 95% CI, which indicated a moderate agreement between the observers.

MBL is among the important factors in the long-term
success of dental implants [5]. Several local and systemic factors are responsible for MBL [6]. The number of implants is considered as an influential factor for the reduction of MBL [2]. In this study, MBL was assessed in   reporting that the MBL decreased with an increase in the number of short implants. The possible reason for the difference in the results of the two studies can be the crown to implant ratio [7]. Another study reported that the crown to implant ratio did not play a role in the increase of MBL [8]. Early MBL is due to the remodeling process of bone, which is not related to infection. Early MBL occurs one year after dental implant placement [9].
Several studies support overloading as a factor responsible for increased MBL [11][12][13]. Occlusal overload is defined as the application of loads greater than the withstanding capability of the implant or prosthetic components or the surrounding tissues [11][12][13]. Minor occlusal overload does not cause MBL [14]. In the placement of two implants with pontic, occlusal overloading does not occur if sufficient bone volume is available, and implants are placed in a correct position.
An excessive dynamic load results in crater-like bone defects lateral to the osseointegrated fixtures [14]. It is unclear whether occlusal overload might be a cause of MBL or not [15]. Moreover, higher remodeling activity of the peri-implant bone occurs around implants under high loading forces [15]. It should be noted that bone quality, implant diameter and implant surface characteristics affect MBL around implants; it is reported that a poor bone quality, a smaller diameter of implants, and a smooth surface adversely affect MBL [16].
In this study, we considered implant diameter and length as the variables of the study. As the implant diameter was not different between the two groups, it cannot be responsible for any possible difference in MBL. It has been reported that narrow fixtures may be associated with higher MBL [17]. Surgeons may have more confidence in the treatment outcome when a higher number of implants are placed. However, financial issues and anatomical limitations may prevent the placement of the maximum number of implants.

Conclusion
It seems that the use of two or three implants for replacement of three missing teeth in the posterior mandible is not associated with an increase in MBL.

Acknowledgments
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Informed consent was obtained from all individual participants included in the study.